MICROPROBE ANALYSIS OF THE GLAUCONITE PELLETS OF THE LIDAM FORMATION NW SIRT BASIN, LIBYA

The glauconite pellets of the Lidam Formation in north-west Sirt Basin, Libya, have been classified into three types. They are grass-green pellets, light-green pellets and brownish-yellow pellets.

The microprobe analysis of the three types shows that the brownish-yellow pellets are the same as grass-green pellets, but the Fe³⁺, Fe²⁺ and K were possibly replaced by Al. These brownish-yellow grains reflect a history of more oxidizing conditions, either through subaerial exposure or transport in shallow turbulent waters during Upper Cenomanian-Lower Turonian low sea level stands. The light-green pellets represent the transition stage between the other two types. It also shows that the mean values of K₂O in the grass-green pellets (glauconite mineral) is 8.7 which reflect localized concentration of K₂O . The roundness of glauconite pellets and their replacement by dolomite suggest that they might have been transported early to the outer ramp. The glauconite pellets are a valuable marine indicator. They occur mainly in relatively shallow water, at depths between 60-500 m. They are particularly characteristic of the inner part of the continental shelf. This highly glauconitic bed of Cenomanian age marks the base of a transgressive sequence.